1. Field of the Invention
The present invention relates to an image pickup apparatus such as a camera that is capable of taking a still image or a moving image and compensating for an exposure amount, a control method therefor, and a computer-readable program for implementing the control method.
2. Description of the Related Art
Conventionally, as a photometric device or an exposure control device used in an image pickup apparatus such as a camera, a technique has been brought into practical use in which an oject field is divided into a plurality of areas, a plurality of pieces of field luminance information are obtained for the respective areas, to obtain a suitable exposure using a predetermined algorithm, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 63-279124, for example.
For example, U.S. Pat. No. 4,523,101 discloses a technique of determining a storage time of an image pickup element in accordance with an output of a photometric sensor for obtaining luminance information of the field.
In many single-lens reflex cameras, a photometric sensor is built in a finder optical system that uses a pentaprism so that some of rays diffused by a diffusing plate called a “focussing screen” become incident on the photometric sensor. In this construction, the photometric sensor is disposed at a location displaced from the optical axis of the finder optical system which is oriented toward an eyepiece lens of the finder.
With the above construction, an amount of light incident on the photometric sensor varies depending on the brightness or speed (F number) of an interchangeable lens attached to the camera, an exit pupil position of, the interchangeable lens, and individual difference such as diffusion characteristics of the focussing screen. As a result, in particular, in the case where luminance information is obtained for the respective divided areas of the photographic screen, there can be a large difference in the light amount balance between a central part and peripheral parts of the screen, which can hinder a determination as to whether the subject is backlit, resulting in an inappropriate photographic exposure being calculated. To fully prevent this, information on the F number unique to the interchangeable lens is inputted from the interchangeable lens to the camera to thereby correct an output signal of the photometric sensor. However, it is difficult to properly correct the signal for every interchangeable lens.
Digital cameras using image pickup devices that have become increasingly widespread in recent years have less latitude for errors in exposure than film cameras, so that even minor errors in exposure exert greater effects upon obtained images than film cameras.